Production of iron whiskers, which are iron filaments of hair-like cross section, by decomposition of iron pentacarbonyl in a magnetic field has been described in U.S. Pat. Nos. 2,884,319 and 3,570,829 (referred to as the Schladitz technique). Formation of iron whiskers by this technique is also reported in U.S. Pat. Nos. 3,441,408, 3,536,519, 3,915,663, 3,915,663, 3,955,962, 4,002,464, and in German Pat. No. 1,224,934. Other U.S. patents (H. J. Schladitz-inventor) involving metallization are U.S. Pat. Nos. 3,770,492, 3,943,221, 4,076,859, 4,096,823, 4,097,624, and 4,108,108.
Iron whiskers were produced by the reaction of iron halides with carbon black (Kittaka and Kaneko, Jap. J. Appl. Phys. 8, 860 (1969)), or iron oxide with coal char (Seaton, Foster, and Valesco, Trans. Iron Steel Inst. Jap. 23, 497 (1983)), or iron oxide with hydrogen (Chemical Abstracts, 78 61165q (1973)) or iron halides with hydrogen (Cochardt and Wiedersich, Naturwissenschaften 42, 342 (1955)). Whiskers of copper, silver, nickel, and cobalt were also grown by hydrogen reduction of the halides (Brenner, Acta Metallurgica, 4, 62 (1956)).
Acicular particles of iron and other metals are produced commercially (Vacuum Metallurgical Co. LTD., Tokyo, Japan) by evaporation of bulk metal into an inert gas under the influence of an applied magnetic field. U.S. Pat. No. 3,206,338 discloses non-pyrophoric, boron-containing acicular ferromagnetic particles.
Metal whiskers have found widespread use as strengthening materials in composites (see U.S. Pat. No. 4,569,886) and, more recently, they have been found to be excellent materials for Electro Magnetic Absorption (EMA) applications. Other uses of acicular metal particles can be found in magnetic recording media, conductive filter media, and catalytic applications. The Schladitz technique has been studied as a means of forming metal whiskers for a number of years. However, a shortcoming of this procedure is that it is a gas phase process requiring high decomposition temperatures employing toxic and flammable metal carbonyls. Such high temperature cannot be used with thermally sensitive substrates. Furthermore, a processing problem in the scale-up occurs due to matting of the whiskers in the reaction chamber so that single, discrete whiskers are not made, rather, bundles are formed. This makes dispersion of the whiskers, especially homogeneous dispersion, difficult, if not impossible.
A. L. Oppegard et al., Journal of Applied Physics, Supplement to Vol. 32, No. 3, 1845-1853 (March 1961) disclose a method of preparing single-domain ferromagnetic particles by borohydride reduction of aqueous solutions of ferromagnetic salts. The fibers contain boron and other trace contaminants including halide.
Ultra fine metal particles and powders having diameters in the range of 50 to 2000 A (0.005 to 0.2 micrometer) are disclosed in the references: A. Tasaki et al., IEEE Transactions on Magnetics, Vol. Mag-19, No. 5, September 1983, pages 1731-1733, and A. Tasaki et al., IEEE Transactions on Magnetics, Vol. Mag-15, No. 6, November 1979, pages 1540-1542.
Slurries of iron particles are disclosed in U.S. Pat. No. 3,228,881. Slurries of chains of iron particles are disclosed in U.S. Pat. No. 3,281,344. There is no mention of slurries of iron fibers or filaments in these references.